Filter cartridges are commonly attached to respirator mask bodies to filter air before it is inhaled by the respirator wearer. Examples of respirator filter cartridges are shown in the following U.S. Pat. No. 7,320,722B2 to Mittelstadt et al., U.S. Pat. No. 7,419,526 to Greer et al., and U.S. Pat. No. 6,277,178 to Holmquist-Brown et al.
Some filter cartridges have an end-of-service-life-indicator, or ESLI, that provides a visible color-change signal to the respirator wearer. This visible signal (sometimes referred to as the “response signal”) indicates when the filter cartridge has met the end of its useful life. Examples of cartridges that use ESLIs are described in the following U.S. Pat. Nos. 4,530,706, 4,326,514, and 4,154,586. In each of these products, the filter cartridges are provided with a transparent sidewall or a shell through which the ESLI may be viewed. ESLI calorimetric sensors, however, often exhibit a progressive color change in the zone of indication. As such, there can be difficulty in assessing when the color change is of such significance that the filter cartridge should be replaced. This difficulty has generally required user training in identifying the particular color change that is indicative of the time to replace the filter cartridge. To address this problem, U.S. Pat. No. 6,497,756 to Curado et al. describes a display window that is defined by an opaque border that surrounds the sensor display field to obscure a predetermined portion of the downstream end of the indicator to provide a desired margin of safety to the wearer in identifying when to change the filter cartridge.
Because the percentage of cartridge life still available at onset of the sensor response signal is strongly dependent on the axial placement of the sensor in the housing, the need for proper ESLI placement adds a significant accuracy requirement to the manufacturing operation. If a sensor is intended to be binary—that is, the need for cartridge replacement occurs at the first visible color change as opposed to an analog-type reading where spatially-progressive color changes may be seen before cartridge replacement—then the sensor must be relatively small, which mandates a significant need for sensor placement accuracy.
An approach to alleviating this drawback has been to use a relatively large sensor inside the housing, about one centimeter or more in characteristic dimension. A mask, such as an adhesive label, is appropriately positioned on the cartridge exterior to overlay the sensor so that only the response signal can be seen through the resulting window. North Safety has sold cartridge RT 41P100, which has had such a construction. Although the label approach allows only for the proper response zone to be witnessed, the placement of adhesive labels to close positional tolerances can sometimes be difficult when attempting to mass manufacture respirator filter cartridges.